Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technique are widely deployed all over the world. High speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with highly competitive radio access technique in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Decrease of cost per bit, increase in service availability, flexible use of frequency bands, simple architecture and open interface, low power consumption of a user equipment and the like are suggested as requirements of next generation communication systems.
Voice over IP (VoIP) is a service that transmits voice data through Internet protocol (IP) and provides voice data over packet switched (PS) domain instead of circuit switched (CS) domain conventionally. The VoIP service transmits connection-less voice data whereas CS based voice service transmits end-to-end voice data, and thus VoIP can use network resources with high efficiency.
With the development of wireless communication techniques, the quantity of user data is increasing very rapidly and PS based service are substituting CS based service for efficient utilization of limited network resources. VoIP is developed in view of this trend and it is expected that most wireless communication systems provide voice services through VoIP.
It is disadvantageous that VoIP has quality of service (QoS) lower than that of the CS based voice service. Typical factors that affect QoS include delay, jitter and a high frame error rate (FER). Although VoIP in the initial developing stage has QoS poorer than the CS based voice service, current VoIP guarantees QoS almost equivalent to the CS base voice service in wired communication.
To effectively provide the PS base voice service, real-time transport protocol (RTP) has been developed, and RTP control protocol (RTCP) for controlling RTP has been also developed. The RTP includes time stamp information in every packet so that a jitter problem can be solved. Furthermore, FER can be reduced through rate control by reporting a loss of RTP packet through the RTCP. A session initiation protocol (SIP) and a session description protocol (SDP) have been developed in addition to RTP/RTCP to maintain end-to-end virtual connection to solve a delay problem.
Though VoIP in wired communication can guarantee satisfactory QoS, VoIP in wireless communication still has QoS considerably poorer than the CS based voice service. Robust header compression (ROHC), which is an improved header compression technique, has been developed and used in order to improve transmission efficiency of VoIP. However, the overall QoS is still lower than that of the CS based voice service.
QoS may be considerably deteriorated when the RTP and RTCP are provided as a single stream because packet properties of RTP and RTCP are different from each other. This is one of the most serious problems to support VoIP in a wireless communication system. The RTP is insensitive to error rate but sensitive to delay and jitter because it is real-time user data whereas the RTCP corresponding to control data is insensitive to delay and jitter but sensitive to error rate. Furthermore, while the RTP carries voice data so that small-size packets are frequently transmitted, the RTCP has large transmission capacity and a low transmission frequency compared to the RTP. The RTCP is irregularly transmitted because it is control data.
A technique for guaranteeing QoS of voice packets is required in order to achieve an efficient VoIP service in a wireless communication system.